Topside Interrogation For Distributed Acoustic Sensing Of Subsea Wells

ABSTRACT

A distributed acoustic system (DAS) with an interrogator, an umbilical line attached at one end to the interrogator, and a downhole fiber attached to the umbilical line at the end opposite the interrogator. A method for optimizing a sampling frequency may begin with identifying a length of a fiber optic cable connected to an interrogator, identifying one or more regions on the fiber optic cable in which a backscatter is received, and optimizing a sampling frequency of a distributed acoustic system (DAS) by identifying a minimum time interval that is between an emission of a light pulse such that at no point in time the backscatter arrives back at the interrogator that corresponds to more than one spatial location along a sensing portion of the fiber optic cable.

BACKGROUND

Boreholes drilled into subterranean formations may enable recovery ofdesirable fluids (e.g., hydrocarbons) using a number of differenttechniques. A number of systems and techniques may be employed insubterranean operations to determine borehole and/or formationproperties. For example, Distributed Acoustic Sensing (DAS) along with afiber optic system may be utilized together to determine borehole and/orformation properties. Distributed fiber optic sensing is acost-effective method of obtaining real-time, high-resolution, highlyaccurate temperature and strain (acoustic) data along the entirewellbore. In examples, discrete sensors, e.g., for sensing pressure andtemperature, may be deployed in conjunction with the fiber optic cable.Additionally, distributed fiber optic sensing may eliminate downholeelectronic complexity by shifting all electro-optical complexity to thesurface within the interrogator unit. Fiber optic cables may bepermanently deployed in a wellbore via single- or dual-trip completionstrings, behind casing, on tubing, or in pumped down installations; ortemporally via coiled tubing, slickline, or disposable cables.

Distributed sensing can be enabled by continuously sensing along thelength of the fiber, and effectively assigning discrete measurements toa position along the length of the fiber via optical time-domainreflectometry (OTDR). That is, knowing the velocity of light in fiber,and by measuring the time it takes the backscattered light to return tothe detector inside the interrogator, it is possible to assign adistance along the fiber.

Distributed acoustic sensing has been practiced for dry-tree wells, buthas not been attempted in wet-tree (or subsea) wells, to enableinterventionless, time-lapse reservoir monitoring via vertical seismicprofiling (VSP), well integrity, flow assurance, and sand control. Asubsea operation requires optical engineering solutions to compensatefor losses accumulated through long (˜5 to 100 km) lengths of subseatransmission fiber, 10 km of in-well subsurface fiber, and multiple wet-and dry-mate optical connectors, splices, and optical feedthroughsystems (OFS).

BRIEF DESCRIPTION OF THE DRAWINGS

For a detailed description of the preferred examples of the disclosure,reference will now be made to the accompanying drawings in which:

FIG. 1 illustrate an example of a well measurement system in a subseaenvironment;

FIG. 2 illustrates an example of a DAS system;

FIG. 3 illustrate an example of a DAS system with lead lines.

FIG. 4 illustrates a schematic of another example DAS system;

FIG. 5 illustrates an example of a remote circulator arrangement;

FIG. 6 illustrates a graph for determining time for a light pulse totravel in a fiber optic cable;

FIG. 7 illustrates another graph for determining time for a light pulseto travel in a fiber optic cable;

FIG. 8 illustrates an example of a remote circulator arrangement;

FIG. 9 illustrates another graph for determining time for a light pulseto travel in a fiber optic cable;

FIG. 10A illustrates a graph of sensing regions in the DAS system;

FIG. 10B illustrates a graph with an active proximal circulator using anoptimized DAS sampling frequency of 12.5 kHz;

FIG. 10C illustrates a graph with a passive proximal circulator using anoptimized DAS sampling frequency of 12.5 kHz;

FIG. 11 illustrates a graph of optimized sampling frequencies in the DASsystem;

FIG. 12 illustrates an example of a workflow for optimizing the samplingfrequencies of the DAS system;

FIG. 13 illustrates another example of the DAS system;

FIG. 14 illustrates another example of the DAS system;

FIG. 15 illustrates another example of the DAS system;

FIG. 16 illustrates an example schematic of an interrogator in the DASsystem;

FIGS. 17A-17D illustrates examples of a downhole fiber deployed in awellbore; and

FIG. 18 illustrates an example of the well measurement system in aland-based operation.

DETAILED DESCRIPTION

The present disclosure relates generally to a system and method forusing fiber optics in a DAS system in a subsea operation. Subseaoperations may present optical challenges which may relate to thequality of the overall signal in the DAS system with a longer fiberoptical cable. The overall signal may be critical since the end of thefiber contains the interval of interest, i.e., the well and reservoirsections. To prevent a drop in signal-to-noise (SNR) and signal quality,the DAS system described below may increase the returned signal strengthwith given pulse power, decrease the noise floor of the receiving opticsto detect weaker power pulses, maintain the pulse power as high aspossible as it propagates down the fiber, increase the number of lightpulses that can be launched into the fiber per second, and/or increasethe maximum pulse power that can be used for given fiber length.

FIG. 1 illustrates an example of a well system 100 that may employ theprinciples of the present disclosure. More particularly, well system 100may include a floating vessel 102 centered over a subterraneanhydrocarbon bearing formation 104 located below a sea floor 106. Asillustrated, floating vessel 102 is depicted as an offshore,semi-submersible oil and gas drilling platform, but could alternativelyinclude any other type of floating vessel such as, but not limited to, adrill ship, a pipelaying slip, a tension-leg platforms (TLPs), a “spar”platform, a production platform, a floating production, storage, andoffloading (FPSO) vessel, and/or the like. Additionally, the methods andsystems described below nays also be utilized on land-based drillingoperations. A subsea conduit or riser 108 extends from a deck 110 offloating vessel 102 to a wellhead. installation 112 that may include oneor more blowout preventers 114. in examples, riser 108 may also bereferred to as a flexible riser, flowline, umbilical, and/or the like.Floating vessel 102 has a hoisting apparatus 116 and a derrick 118 forraising and lowering tubular lengths of drill pipe, such as a tubular120. In examples, tubular 120 may be a drill string, casing, productionpipe, and/or the like.

A wellbore 122 extends through the various earth strata toward thesubterranean hydrocarbon bearing formation 104 and tubular 120 may beextended within wellbore 122, Even though. FIG. 1 depicts a verticalwellbore 122, it should. be understood by those skilled in the art thatthe methods and systems described are equally well suited for use inhorizontal or deviated wellbores. During drilling operations, the distalend of tubular 120, for example a drill sting, may include a bottom holeassembly (BHA) that includes a drill bit and a downhole drilling motor,also referred to as a positive displacement motor (“PDM”) or “mudmotor.” During production operations, tubular 120 may include a DASsystem. The DAS system may be inclusive of an interrogator 124,umbilical line 126, and downhole fiber 128.

Downhole fiber 128 may be permanently deployed in a wellbore via single-or dual-trip completion strings, behind casing, on tubing, or in pumpeddown installations. In examples, downhole fiber 128 may be temporarilydeployed via coiled tubing, wireline, slickline, or disposable cables.FIGS. 17A-17D illustrate examples of different types of deployment ofdownhole fiber 128 in wellbore 122 (e.g., referring to FIG. 1), Asillustrated in FIG. 17A, wellbore 122 deployed in formation 104 mayinclude surface casing 1700 in which production casing 1702 may bedeployed. Additionally, production tubing 1704 may be deployed withinproduction casing 1702. In this example, downhole fiber 128 may betemporarily deployed in a wireline system in which a bottom hole gauge1708 is connected to the distal end of downhole fiber 128. Furtherillustrated, downhole fiber 128 may be coupled to a fiber connection1706. Without limitation, fiber connection 1706 may attach downholefiber 128 to umbilical line 126 (e.g., referring to FIG. 1). Fiberconnection 1706 may operate with an optical feedthrough system (itselfcomprising a series of wet- and dry-mate optical connectors) in thewellhead that optically couples downhole fiber 128 from the tubinghanger, to umbilical line 126 on the wellhead. instrument panel.Umbilical line 126 may consist of an optical flying lead, opticaldistribution system(s), umbilical termination unit(s), and transmissionfibers encapsulated in flying leads, flow lines, rigid risers, flexiblerisers, and/or one or more umbilical lines. This may allow for umbilicalline 126 to connect and disconnect from downhole fiber 128 whilepreserving optical continuity between the umbilical line 126 and thedownhole fiber 128.

FIG. 17B illustrates an example of permanent deployment of downholefiber 128. As illustrated in wellbore 122 deployed in formation 104 mayinclude surface casing 1700 in which production casing 1702 may bedeployed. Additionally, production tubing 1704 may be deployed withinproduction casing 1702. In examples, downhole fiber 128 is attached tothe outside of production tubing 1704 by one or more cross-couplingprotectors 1710. Without limitation, cross-coupling protectors 1710 maybe evenly spaced and may be disposed on every other joint of productiontubing 1704. Further illustrated, downhole fiber 128 may be coupled tofiber connection 1706 at one end and bottom hole gauge 1708 at theopposite end.

FIG. 17C illustrates an example of permanent deployment of downholefiber 128. As illustrated in wellbore 122 deployed in formation 104 mayinclude surface casing 1700 in which production casing 1702 may bedeployed. Additionally, production tubing 1704 may be deployed withinproduction casing 1702. In examples, downhole fiber 128 is attached tothe outside of production casing 1702 by one or more cross-couplingprotectors 1710. Without limitation, cross-coupling protectors 1710 maybe evenly spaced and may be disposed on every other joint of productiontubing 1704. Further illustrated, downhole fiber 128 may be coupled tofiber connection 1706 at one end and bottom hole gauge 1708 at theopposite end.

FIG. 17D illustrates an example of a coiled tubing operation in whichdownhole fiber 128 may be deployed temporarily. As illustrated in FIG.17D, wellbore 122 deployed in formation 104 may include surface casing1700 in which production casing 1702 may be deployed. Additionally,coiled tubing 1712 may be deployed within production casing 1702. Inthis example, downhole fiber 128 may be temporarily deployed in a coiledtubing system in which a bottom hole gauge 1708 is connected to thedistal end of downhole fiber. Further illustrated, downhole fiber 128may be attached to coiled tubing 1712, which may move downhole fiber 128through production casing' 1702. Further illustrated, downhole fiber 128may be coupled to fiber connection 1706 at one end and bottom hole gauge1708 at the opposite end. During operations downhole fiber 128 may beused to take measurements within wellbore 122, which may be transmittedto the surface and/or interrogator 124 (e.g., referring to FIG. 1) inthe DAS system.

Additionally, within the DAS system, interrogator 124 may be connectedto an information handling system 130 through connection 132, which maybe wired and/or wireless. It should be noted that both informationhandling system 130 and interrogator 124 are disposed on floating vessel102. Both systems and methods of the present disclosure may beimplemented, at least in part, with information handling system 130.Information handling system 130 may include any instrumentality oraggregate of instrumentalities operable to compute, estimate, classify,process, transmit, receive, retrieve, originate, switch, store, display,manifest, detect, record, reproduce, handle, or utilize any form ofinformation, intelligence, or data for business, scientific, control, orother purposes. For example, an information handling system 130 may be aprocessing unit 134, a network storage device, or any other suitabledevice and may vary in size, shape, performance, functionality, andprice. Information handling system 130 may include random access memory(RAM), one or more processing resources such as a central processingunit (CPU) or hardware or software control logic, ROM, and/or othertypes of nonvolatile memory. Additional components of the informationhandling system 130 may include one or more disk drives, one or morenetwork ports for communication with external devices as well as aninput device 136 (e.g., keyboard, mouse, etc.) and video display 138.Information handling system 130 may also include one or more busesoperable to transmit communications between the various hardwarecomponents.

Alternatively, systems and methods of the present disclosure may beimplemented, at least in part, with non-transitory computer-readablemedia 140. Non-transitory computer-readable media 140 may include anyinstrumentality or aggregation of instrumentalities that may retain dataand/or instructions for a period of time. Non-transitorycomputer-readable media 140 may include, for example, storage media suchas a direct access storage device (e.g., a hard disk drive or floppydisk drive), a sequential access storage device (e.g., a tape diskdrive), compact disk, CD-ROM, DVD, RAM, ROM, electrically erasableprogrammable read-only memory (EEPROM), and/or flash memory; as well ascommunications media such as wires, optical fibers, microwaves, radiowaves, and other electromagnetic and/or optical carriers; and/or anycombination of the foregoing.

Production operations in a subsea environment present optical challengesfor DAS. For example, a maximum pulse power that may be used in DAS isapproximately inversely proportional to fiber length due to opticalnon-linearities in the fiber. Therefore, the quality of the overallsignal is poorer with a longer fiber than a shorter fiber. This mayimpact any operation that may utilize the DAS since the distal end ofthe fiber actually contains the interval of interest (i.e., thereservoir) in which downhole fiber 128 may be deployed. The interval ofinterest may include wellbore 122 and formation 104. For pulsed DASsystems such as the one exemplified in FIG. 2, an additional challengeis the drop-in signal to noise ratio (SNR) associated with the decreasein the number of light pulses that may be launched into the fiber persecond (pulse rate) when interrogating fibers with overall lengthsexceeding 10 km. As such, utilizing DAS in a subsea environment may haveto increase the returned signal strength with given pulse power,increase the maximum pulse power that may be used for given fiber opticcable length, maintain the pulse power as high as possible as itpropagates down the fiber optic cable length, and increase the number oflight pulses that may be launched into the fiber optic cable per second.

FIG. 18 illustrates an example of a land-based well system 1800, whichillustrates a coiled tubing operation. Without limitation, while acoiled tubing operation is shown, a wireline operation and/or the likemay be utilized. As illustrated interrogator 124 is attached toinformation handling system 130. Further discussed below, lead lines mayconnect umbilical line 126 to interrogator 124. Umbilical line 126 mayinclude a first fiber optic cable 304 and a second fiber optic cable 308which may be individual lead lines. Without limitation, first fiberoptic cable 304 and a second fiber optic cable 308 may attach to coiledtubing 1802 as umbilical line 126. Umbilical line 126 may traversethrough wellbore 122 attached to coiled tubing 1802. In examples, coiledtubing 1802 may be spooled within hoist 1804. Hoist 1804 may be used toraise and/or lower coiled tubing 1802 in wellbore 122. Furtherillustrated in FIG. 18, umbilical line 126 may connect to distalcirculator 312, further discussed below. Distal circulator 312 mayconnect umbilical line 126 to downhole fiber 128.

FIG. 2 illustrates an example of DAS system 200. DAS system 200 mayinclude information handling system 130 that is communicatively coupledto interrogator 124. Without limitation, DAS system 200 may include asingle-pulse coherent Rayleigh scattering system with a compensatinginterferometer. In examples, DAS system 200 may be used for phase-basedsensing of events in a wellbore using measurements of coherent Rayleighbackscatter or may interrogate a fiber optic line containing an array ofpartial reflectors, for example, fiber Bragg gratings.

As illustrated in FIG. 2, interrogator 124 may include a pulse generator214 coupled to a first coupler 210 using an optical fiber 212. Pulsegenerator 214 may be a laser, or a laser connected to at least oneamplitude modulator, or a laser connected to at least one switchingamplifier, i.e., semiconductor optical amplifier (SOA). First coupler210 may be a traditional fused type fiber optic splitter, a circulator,a PLC fiber optic splitter, or any other type of splitter known to thosewith ordinary skill in the art. Pulse generator 214 may be coupled tooptical gain elements (not shown) to amplify pulses generated therefrom.Example optical gain elements include, but are not limited to, ErbiumDoped Fiber Amplifiers (EDFAs) or Semiconductor Optical Amplifiers(SOAs).

DAS system 200 may include an interferometer 202. Without limitations,interferometer 202 may include a Mach-Zehnder interferometer. Forexample, a Michelson interferometer or any other type of interferometer202 may also be used without departing from the scope of the presentdisclosure. Interferometer 202 may include a top interferometer arm 224,a bottom interferometer arm 222, and a gauge 223 positioned on bottominterferometer arm 222. Interferometer 202 may be coupled to firstcoupler 210 through a second coupler 208 and an optical fiber 232.Interferometer 202 further may be coupled to a photodetector assembly220 of DAS system 200 through a third coupler 234 opposite secondcoupler 208. Second coupler 208 and third coupler 234 may be atraditional fused type fiber optic splitter, a PLC fiber optic splitter,or any other type of optical splitter known to those with ordinary skillin the art. Photodetector assembly 220 may include associated optics andsignal processing electronics (not shown). Photodetector assembly 220may be a semiconductor electronic device that uses the photoelectriceffect to convert light to electricity. Photodetector assembly 220 maybe an avalanche photodiode or a pin photodiode but is not intended to belimited to such.

When operating DAS system 200, pulse generator 214 may generate a firstoptical pulse 216 which is transmitted through optical fiber 212 tofirst coupler 210. First coupler 210 may direct first optical pulse 216through a fiber optical cable 204. It should be noted that fiber opticalcable 204 may be included in umbilical line 126 and/or downhole fiber128 (e.g., FIG. 1). As illustrated, fiber optical cable 204 may becoupled to first coupler 210. As first optical pulse 216 travels throughfiber optical cable 204, imperfections in fiber optical cable 204 maycause a portion of the light to be backscattered along fiber opticalcable 204 due to Rayleigh scattering. Scattered light according toRayleigh scattering is returned from every point along fiber opticalcable 204 along the length of fiber optical cable 204 and is shown asbackscattered light 228 in FIG. 2. This backscatter effect may bereferred to as Rayleigh backscatter. Density fluctuations in fiberoptical cable 204 may give rise to energy loss due to the scatteredlight, α_(scat), with the following coefficient:

$\begin{matrix}{\alpha_{scat} = {\frac{8\pi^{3}}{3\lambda^{4}}n^{8}p^{2}kT_{f}\beta}} & (1)\end{matrix}$

where n is the refraction index, p is the photoelastic coefficient offiber optical cable 204, k is the Boltzmann constant, and β is theisothermal compressibility. T_(f) is a fictive temperature, representingthe temperature at which the density fluctuations are “frozen” in thematerial. Fiber optical cable 204 may be terminated with a lowreflection device (not shown). In examples, the low reflection device(not shown) may be a fiber coiled and tightly bent to violate Snell'slaw of total internal reflection such that all the remaining energy issent out of fiber optical cable 204.

Backscattered light 228 may travel back through fiber optical cable 204,until it reaches second coupler 208. First coupler 210 may be coupled tosecond coupler 208 on one side by optical fiber 232 such thatbackscattered light 228 may pass from first coupler 210 to secondcoupler 208 through optical fiber 232. Second coupler 208 may splitbackscattered light 228 based on the number of interferometer arms sothat one portion of any backscattered light 228 passing throughinterferometer 202 travels through top interferometer arm 224 andanother portion travels through bottom interferometer arm 222.Therefore, second coupler 208 may split the backscattered light fromoptical fiber 232 into a first backscattered pulse and a secondbackscattered pulse. The first backscattered pulse may be sent into topinterferometer arm 224. The second backscattered pulse may be sent intobottom interferometer arm 222. These two portions may be re-combined atthird coupler 234, after they have exited interferometer 202, to form aninterferometric signal.

Interferometer 202 may facilitate the generation of the interferometricsignal through the relative phase shift variations between the lightpulses in top interferometer arm 224 and bottom interferometer arm 222.Specifically, gauge 223 may cause the length of bottom interferometerarm 222 to be longer than the length of top interferometer arm 224. Withdifferent lengths between the two arms of interferometer 202, theinterferometric signal may include backscattered light from twopositions along fiber optical cable 204 such that a phase shift ofbackscattered light between the two different points along fiber opticalcable 204 may be identified in the interferometric signal. The distancebetween those points L may be half the length of the gauge 223 in thecase of a Mach-Zehnder configuration, or equal to the gauge length in aMichelson interferometer configuration.

While DAS system 200 is running, the interferometric signal willtypically vary over time. The variations in the interferometric signalmay identify strains in fiber optical cable 204 that may be caused, forexample, by seismic energy. By using the time of flight for firstoptical pulse 216, the location of the strain along fiber optical cable204 and the time at which it occurred may be determined. If fiberoptical cable 204 is positioned within a wellbore, the locations of thestrains in fiber optical cable 204 may be correlated with depths in theformation in order to associate the seismic energy with locations in theformation and wellbore.

To facilitate the identification of strains in fiber optical cable 204,the interferometric signal may reach photodetector assembly 220, whereit may be converted to an electrical signal. The photodetector assemblymay provide an electric signal proportional to the square of the sum ofthe two electric fields from the two arms of the interferometer. Thissignal is proportional to:

P(t)=P1+P2+2*√{square root over ((P1P2)cos(ϕ1−ϕ2))}  (2)

where P_(n) is the power incident to the photodetector from a particulararm (1 or 2) and ϕ_(n) is the phase of the light from the particular armof the interferometer. Photodetector assembly 220 may transmit theelectrical signal to information handling system 130, which may processthe electrical signal to identify strains within fiber optical cable 204and/or convey the data to a display and/or store it in computer-readablemedia. Photodetector assembly 220 and information handling system 130may be communicatively and/or mechanically coupled. Information handlingsystem 130 may also be communicatively or mechanically coupled to pulsegenerator 214.

Modifications, additions, or omissions may be made to FIG. 2 withoutdeparting from the scope of the present disclosure. For example, FIG. 2shows a particular configuration of components of DAS system 200.However, any suitable configurations of components may be used. Forexample, pulse generator 214 may generate a multitude of coherent lightpulses, optical pulse 216, operating at distinct frequencies that arelaunched into the sensing fiber either simultaneously or in a staggeredfashion. For example, the photo detector assembly is expanded to featurea dedicated photodetector assembly for each light pulse frequency. Inexamples, a compensating interferometer may be placed in the launch path(i.e., prior to traveling down fiber optical cable 204) of theinterrogating pulse to generate a pair of pulses that travel down fiberoptical cable 204. In examples, interferometer 202 may not be necessaryto interfere the backscattered light from pulses prior to being sent tophoto detector assembly. In one branch of the compensationinterferometer in the launch path of the interrogating pulse, an extralength of fiber not present in the other branch (a gauge length similarto gauge 223 of FIG. 1) may be used to delay one of the pulses. Toaccommodate phase detection of backscattered light using DAS system 200,one of the two branches may include an optical frequency shifter (forexample, an acousto-optic modulator) to shift the optical frequency ofone of the pulses, while the other may include a gauge. This may allowusing a single photodetector receiving the backscatter light todetermine the relative phase of the backscatter light between twolocations by examining the heterodyne beat signal received from themixing of the light from different optical frequencies of the twointerrogation pulses.

In examples, DAS system 200 may generate interferometric signals foranalysis by the information handling system 130 without the use of aphysical interferometer. For instance, DAS system 200 may directbackscattered light to photodetector assembly 220 without first passingit through any interferometer, such as interferometer 202 of FIG. 2.Alternatively, the backscattered light from the interrogation pulse maybe mixed with the light from the laser originally providing theinterrogation pulse. Thus, the light from the laser, the interrogationpulse, and the backscattered signal may all be collected byphotodetector assembly 220 and then analyzed by information handlingsystem 130. The light from each of these sources may be at the sameoptical frequency in a homodyne phase demodulation system, or may bedifferent optical frequencies in a heterodyne phase demodulator. Thismethod of mixing the backscattered light with a local oscillator allowsmeasuring the phase of the backscattered light along the fiber relativeto a reference light source.

FIG. 3 illustrates an example of DAS system 200, which may be utilizedto overcome challenges presented by a subsea environment. DAS system 200may include interrogator 124, umbilical line 126. and downhole fiber128. As illustrated, interrogator 124 may include pulse generator 214and photodetector assembly 220, both of which may be communicativelycoupled to information handling system 130. Additionally,interferometers 202 may be placed within interrogator 124 and operateand/or function as described above. FIG. 3 illustrates an example of DASsystem 200 in which lead lines 300 may be used. As illustrated, anoptical fiber 212 may attach pulse generator 214 to an output 302, whichmay be a fiber optic connector. Umbilical line 126 may attach to output302 with a first fiber optic cable 304. First fiber optic cable 304 martraverse the length of umbilical line 126 to a remote circulator 306.Remote circulator 306 may connect first fiber optic cable 304 to secondfiber optic cable 308. In examples, remote circulator 306 functions tosteer light unidirectionally between one or more input and outputs ofremote circulator 306. Without limitation, remote circulators 306 arethree-port devices wherein light from a first port is split internallyinto two independent polarization states and wherein these twopolarization states are made to propagate two different paths insideremote circulator 306. These two independent paths allow one or bothindependent light beams to be rotated in polarization state via theFaraday effect in optical media. Polarization rotation of the lightpropagating through free space optical elements within the circulatorthus allows the total optical power of the two independent beams touniquely emerge together with the same phase relationship from a secondport of remote circulator 306.

Conversely, if any light enters the second port of remote circulator 306in the reverse direction, the internal free space optical elementswithin remote circulator 306 may operate identically on the reversedirection light to split it into two polarizations states. Afterappropriate rotation of polarization states, these reverse in directionpolarized light beams, are recombined, as in the forward propagationcase, and emerge uniquely from a third port of remote circulator 306with the same phase relationship and optical power as they had beforeentering remote circulator 306. Additionally, as discussed below, remotecirculator 306 may act as a gateway, which may only allow chosenwavelengths of light to pass through remote circulator 306 and pass todownhole fiber 128. Second fiber optic cable 308 may attach umbilicalline 126 to input 309. Input 309 may be a fiber optic connector whichmay allow backscatter light to pass into interrogator 124 tointerferometer 202. Interferometer 202 may operate and function asdescribed above and further pass back scatter light to photodetectorassembly 220.

FIG. 4 illustrates another example of DAS system 200. As illustrated,interrogator 124 may include one or more DAS interrogator units 400,each emitting coherent light pulses at a distinct optical wavelength,and a Raman Pump 402 connected to a wavelength division multiplexer 404(WDM) with fiber stretcher. Without limitation, WDM 404 may include amultiplexer assembly that multiplexes the light received from the one ormore DAS interrogator units 400 and a Raman Pump 402 onto a singleoptical fiber and a demultiplexer assembly that separates themulti-wavelength backscattered light into its individual frequencycomponents and redirects each single-wavelength backscattered lightstream back to the corresponding DAS interrogator unit 400. In anexample, WDM 404 may utilize an optical add-drop multiplexer to enablemultiplexing the light received from the one or more DAS interrogatorunits 400 and a Raman Pump 402 and demultiplexing the multi-wavelengthbackscattered light received from a single fiber. WDM 404 may alsoinclude circuitry to optically amplify the multi-frequency light priorto launching it into the signal optical fiber and/or optical circuitryto optically amplify the multi-frequency backscattered light returningfrom the single optical fiber, thereby compensating for optical lossesintroduced during optical (de-)multiplexing. Raman Pump 402 may be aco-propagating optical pump based on stimulated Raman scattering, tofeed energy from a pump signal to a main pulse from one or more DASinterrogator units 400 as the main pulse propagates down one or morefiber optic cables. This may conservatively yield a 3 dB improvement inSNR. As illustrated, Raman Pump 402 is located in interrogator 124 forco-propagation, in another example, Raman Pump 402 may be locatedtopside after one or more remote circulators 306 either in line withfirst fiber optic cable 304 (co-propagation mode) and/or in line withsecond fiber optic cable 308 (counter-propagation). In another example,Raman Pump 402 is marinized and located after distal circulator 312configured either for co-propagation or counter-propagation. In stillanother example, the light emitted by the Raman Pump 402. is remotelyreflected by using a wavelength-selective filter beyond a remotecirculator in order to provide amplification in the return path using aRaman Pump 402 in any of the topside configurations outlined above.

Further illustrated in FIG. 4. WDM 404 with fiber stretcher may attachproximal circulator 310 to umbilical line 126. Umbilical line 126 mayinclude one or more remote circulators 306, a first fiber optic cable304, and a second fiber optic cable 308. As illustrated, a first fiberoptic cable 304 and as second fiber optic cable 308 may be separate andindividual fiber optic cables that may be attached at each end to one ormore remote circulators 306. In examples, first fiber optic cable 304and second fiber optic cable 308 may be different lengths or the samelength and each may be an ultra-low loss transmission fiber that mayhave a higher power handling capability before non-literarily. This mayenable a higher gain, co-propagation Raman amplification frominterrogator 124.

Deploying first fiber optic cable 304 and as second fiber optic cable308 from floating vessel 102 (e.g., referring to FIG. 1) to a subseaenvironment to a distal-end passive optical circulator arrangement,enables downhole fiber 128, which is a sensing fiber, to be below aremote circulator 306 (e.g., well-only) that may be at the distal end ofDAS system 200. Higher (2-3×) pulse repetition rates, and non-saturated(non-back reflected) optical receivers may also be adjusted such thattheir dynamic range is optimized for downhole fiber 128. This mayapproximately yield a 3.5 dB improvement in SNR. Additionally, downholefiber 128 may be a sensing fiber that has higher Rayleigh scatteringcoefficient (i.e., higher doping) which may be result in a ten timesimprovement in backscatter, which may yield a 7-dB improvement in SNR.In examples, remote circulators 306 may further be categorized as aproximal circulator 310 and a distal circulator 312. Proximal circulator310 is located closer to interrogator 124 and may be located on floatingvessel 102 or within umbilical line 126. Distal circulator 312 may befurther away from interrogator 124 than proximal circulator 310 and maybe located in umbilical line 126 or within wellbore 122 (e.g., referringto FIG. 1). As discussed above, a configuration illustrated in FIG. 3may not utilize a proximal circulator 310 with lead lines 300.

FIG. 5 illustrates another example of distal circulator 312, which mayinclude two remote circulators 306. As illustrated, each remotecirculator 306 may function and operate to avoid overlap, atinterrogator 124, of backscattered light from two different pulses. Forexample, during operations, light at a first wavelength may travel frominterrogator 124 down first fiber optic cable 304 to a remote circulator306. As the light passes through remote circulator 306 the light mayencounter a Fiber Bragg Grating 500. In examples, Fiber Bragg Grating500 may be referred to as a filter mirror that may be a wavelengthspecific high reflectivity filter mirror or filter reflector that mayoperate and function to recirculate unused light back through theoptical circuit for “double-pass” co/counter propagation induced DASsignal gain at 1550 nm. In examples, this wavelength specific “Ramanlight” mirror may be a dichroic thin film interference filter, FiberBragg Grating 500, or any other suitable optical filter that passes onlythe 1550 nm forward propagating DAS interrogation pulse light whilesimultaneously reflecting most of the residual Raman Pump light.

Without limitation, Fiber Bragg Grating 500 may be set-up, fabricated,altered, and/or the like to allow only certain selected wavelengths oflight to pass. All other wavelengths may be reflected back to the secondremote circulator, which may send the reflected wavelengths of lightalong second fiber optic cable 308 back to interrogator 124. This mayallow Fiber Bragg Grating 500 to split DAS system 200 (e.g., referringto FIG. 4) into two regions. A first region may be identified as thedevices and components before Fiber Bragg Grating 500 and the secondregion may be identified as downhole fiber 128 and any other devicesafter Fiber Bragg Grating 500.

Splitting DAS system 200 (e.g., referring to FIG. 4) into two separateregions may allow interrogator 124 (e.g., referring to FIG. 1) to pumpspecifically for an identified region. For example, the disclosed systemof FIG. 4 may include one or more pumps, as described above, placed ininterrogator 124 or after proximal circulator 310 at the topside eitherin line with first fiber optic cable 304 or second fiber optic cable 308that may emit a wavelength of light that may travel only to a firstregion and be reflected by Fiber Bragg Grating 500. A second pump mayemit a wavelength of light that may travel to the second region bypassing through Fiber Bragg Grating 500. Additionally, both the firstpump and second pump may transmit at the same time. Without limitation,there may be any number of pumps and any number of Fiber Bragg Gratings500 which may be used to control what wavelength of light travelsthrough downhole fiber 128. FIG. 5 also illustrates Fiber Bragg Gratings500 operating in conjunction with any remote circulator 306, whether itis a distal circulator 312 or a proximal circulator 310. Additionally,as discussed below, Fiber Bragg Gratings 500 may be attached at thedistal end of downhole fiber 218. Other alterations to DAS system 200(e.g., referring to FIG. 4) may be undertaken to improve the overallperformance of DAS system 200. For example, the lengths of first fiberoptic cable 304 and second fiber optic cable 308 selected to increasepulse repetition rate (expressed in terms of the time interval betweenpulses t_(rep)).

FIG. 6 illustrates an example of fiber optic cable 600 in which noremote circulator 306 may be used. As illustrated, the entire fiberoptic cable 600 is a sensor and the pulse interval must be greater thanthe time for the pulse of light to travel to the end of fiber opticcable 600 and its backscatter to travel back to interrogator 124 (e.g.,referring to FIG. 1). This is so, since in DAS systems 200 at no pointin time, backscatter from more than one location along sensing fiber(i.e., downhole fiber 128) may be received. Therefore, the pulseinterval t_(rep) must be greater than twice the time light takes totravel “one-way” down the fiber. Let is be the “two-way” time for lightto travel to the end of fiber optic cable 600 and back, which may bewritten as t_(rep)>t_(s).

FIG. 7 illustrates an example of fiber optic cable 600 with a remotecirculator 306 using the configuration shown in FIG. 3. When a remotecirculator 306 is used, only the light traveling in fiber optic cable600 that is allowed to go beyond remote circulator 306 and to downholefiber 128 may be returned to interrogator 124 (e.g., referring to FIG.1), thus, the interval between pulses is dictated only by the length ofthe sensing portion, downhole fiber 128, of fiber optic cable 600. Itshould be noted that all light must travel “to” and “from” the sensingportion, downhole fiber 128, with respect to pulse timing, what mattersis the total length of fiber “to” and “from” remote circulator 306.Therefore, first fiber optic cable 304 or second fiber optic cable 308may be longer than the other, as discussed above.

FIG. 8 illustrates an example remote circulator arrangement 800 whichmay allow, as described above, configurations that use more than oneremote circulator 306 close together at the remote location. Althoughremote circulator arrangement 800 may have any number of remotecirculators 306, remote circulator arrangement 800 may be illustrated asa single remote circulator 306.

FIG. 9 illustrates an example first fiber optic cable 304 and secondfiber optic cable 308 attached to a remote circulator 306 at each end.As discussed above, each remote circulator may be categorized as aproximal circulator 310 and a distal circulator 312. When using aproximal circulator 310 and a distal circulator 312, light from thefiber section before proximal circulator 310, and light from the fibersection below the remote circular 306 are detected, which is illustratedin FIGS. 10 and 11. There is a gap 1000 between them of “no light” thatdepends on the total length of fiber (summed) between proximalcirculator 310 and a distal circulator 312.

Referring back to FIG. 9, with t_(s1) the duration of the light fromfiber sensing section before proximal circulator 310, t_(sep) the “deadtime” separating the two sections (and due to the cumulative length offirst fiber optic cable 304 and second fiber optic cable 308 betweenproximal circulator 310 and a distal circulator 312), and t_(s2) theduration of the light from the sensing fiber, downhole fiber 128, beyonddistal circulator 312, the constraints on fiber lengths and pulseintervals may be identified as:

i. t_(rep)<t_(sep)   (3)

ii. (2t _(rep))>(t _(s1) +t _(sep) +t _(s2))   (4)

Criterion (i) ensures that “pulse n” light from downhole fiber 128 doesnot appear while “pulse n+1” light from fiber before proximal circulator310 is being received at interrogator 124 (e.g., referring to FIG. 1).Criterion (ii) ensures that “pulse n” light from downhole fiber 128 isfully received before “pulse n+2” light from fiber before proximalcirculator 310 is being received at interrogator 124 is received. Itshould be noted that the two criteria given above only define theminimum and maximum t_(rep) for scenarios where two pulses are launchedin the fiber before backscattered light below the remote circulator 306is received. However, it should be appreciated that for those skilled inthe art these criteria maybe generalized to cases where n∈{1,2,3 . . . }light pulses may be launched in the fiber before backscattered lightbelow the remote circulator 306 is received.

The use of remote circulators 306 may allow for DAS system 200 (e.g.,referring to FIG. 3) to increase the sampling frequency. FIG. 12illustrates workflow 1200 for optimizing sampling frequency when using aremote circulator 306 in DAS system 200. Workflow 1200 may begin withblock 1202, which determines the overall fiber length in bothdirections. For example, a 17 km of first fiber optic cable 304 and 17km of second fiber optic cable 308 before distal circulator 312 and 8 kmof sensing fiber, downhole fiber 128, after distal circulator 312, theoverall fiber optic cable length in both directions would be 50 km.Assuming a travel time of the light of 5 ns/m, the following equationmay be used to calculate a first DAS sampling frequency f_(s).

$\begin{matrix}{f_{s} = {\frac{1}{t_{s}} = \frac{1}{{5 \cdot 10^{- 9}}z}}} & (5)\end{matrix}$

where t_(s) is the DAS sampling interval and z is the overall two-wayfiber length. Thus, for an overall two way fiber length of 50 km thefirst DAS sampling rate f_(s) is 4 kHz. In block 1204 regions of thefiber optic cable are identified for which backscatter is received. Forexample, this is done by calculating the average optical backscatteredenergy for each sampling location followed by a simple thresholdingscheme. The result of this step is shown in FIG. 10A where boundaries1002 identify two sensing regions 1004. As illustrated in FIG. 10,optical energy is given as:

I²+Q²   (6)

where I and Q correspond to the in-phase (I) and quadrature (Q)components of the backscattered light. In block 1206, the samplingfrequency of DAS system 200 is optimized. To optimize the samplingfrequency a minimum time interval is found that is between the emissionof light pulses such that at no point in time backscattered lightarrives back at interrogator 124 (e.g., referring to FIG. 1) thatcorresponds to more than one spatial location along a sensing portion ofthe fiber-optic line. Mathematically, this may be defined as follows.Let S be the set of all spatial sample locations x along the fiber forwhich backscattered light is received. The desired light pulse emissioninterval t_(s) is the smallest one for which the cardinality of the twosets S and {mod(x, t_(s)):x∈S} is still identical, which is expressedas:

$\begin{matrix}{{\min\limits_{t_{s}}{\left( t_{s} \right)\mspace{14mu} {s.t.\mspace{14mu} {S}}}} = {\left\{ {{{{mod}\left( {x,t_{s}} \right)}\text{:}x} \in S} \right\} }} & (7)\end{matrix}$

where |·|is the cardinality operator, measuring the number of elementsin a set. FIG. 11 shows the result of optimizing the sampling frequencyfrom FIG. 10 with workflow 1200. Here, the DAS sampling frequency mayincrease from 4 kHz to 12.5 kHz without causing any overlap inbackscattered locations, effectively increasing the signal to noiseratio of the underlying acoustic data by more than 5 dB due to theincrease in sampling frequency.

Variants of DAS system 200 may also benefit from workflow 1200. Forexample, FIG. 13 illustrates DAS system 200 in which proximal circulator310 is placed within interrogator 124. This system set up of DAS system200 may allow for system flexibility on how to implement duringmeasurement operations and the efficient placement of Raman Pump 402. Asillustrated in FIGS. 13 and 14, first fiber optic cable 304 and secondfiber optic cable 308 may connect interrogator 124 to umbilical line126, which is described in greater detail above in FIG. 3.

FIG. 14 illustrates another example of DAS system 200 in which RamanPump 402 is operated in co-propagation mode and is attached to firstfiber optic cable 304 after proximal circulator 310. For example, if thefirst sensing region before proximal circulator 310 should not beaffected by Raman amplification. Moreover, Raman Pump 402, may also beattached to second fiber optic cable 308 which may allow the Raman Pump402 to be operated in counter-propagation mode. In examples, the RamanPump may also be attached to fiber 1400 between WDM 404 and proximalcirculator 310 in interrogator 124.

FIG. 15 illustrates another example of DAS system 200 in which anoptical amplifier assembly 1500 (i.e., an Erbium doped fiber amplifier(EDFA)+Fabry-Perot filter) may be attached to proximal circulator 310,which may also be identified as a proximal locally pumped opticalamplifier. In examples, a distal optical amplifier assembly 1502 mayalso be attached at distal circulator 312 on first fiber optical cable304 or second fiber optical cable 308 as an inline or “mid-span”amplifier. In examples, optical amplifier assembly 1502 located in-linewith fiber optical cable 304 and above distal circulator 312 may be usedto boost the light pulse before it is launched into the downhole fiber128. Referring to FIGS. 10B and 10C, the effect of using an opticalamplifier assembly 1500 in-line with a second fiber optic cable 308prior to proximal circulator 310 and/or using an distal opticalamplifier assembly 1502 located in line with second fiber optical cable308 above distal circulator 312 may allow for selectively amplifying thebackscattered light originating from downhole fiber 128 which tends tosuffer from much stronger attenuation as it travels back along downholefiber 128 and second fiber optical cable 308 than backscattered lightoriginating from shallower sections of fiber optic cable that may alsoperform sensing functions. FIG. 10B illustrates measurements whereproximal circulator 310 is active (optical amplifier assembly 1500in-line with a second fiber optic cable 308 prior to proximal circulator310 and/or distal optical amplifier assembly 1502 located in line withsecond fiber optical cable 308 above distal circulator 312 is used).FIG. 10C illustrates measurements where proximal circulator 310 ispassive (no optical amplification is used in-line with second fiberoptic cable 308). In FIGS. 10B and 10C, boundaries 1002 identify twosensing regions 1004. Additionally, in FIGS. 10B and 10C the DASsampling frequency is set to 12.5 kHz using workflow 1200. Furtherillustrated Fiber Bragg Grating 500 may also be disposed on first fiberoptical cable 304 between distal optical amplifier assembly 1502 anddistal circulator 312.

FIG. 16 illustrates an example schematic view of interrogator 124. Asillustrated interrogator 124 may be connected to umbilical line 126 anddownhole fiber 128 to form DAS system 200. As illustrated, umbilicalline 126 may include any number of distal circulators 312 and downholefiber 128 may include an optional Raman Mirror, which may also bereferred to as Fiber Bragg Grating 500.

Interrogator 124 may include one or more lasers 1600. Lasers 1600 may bemultiplexing laser, which may operate by multiplexing a pluralitycoherent laser sources via a WDM 404. One or more lasers 1600 may emit alight pulse 1602, which may be of a modified pulse shape. Optical pulseshaping and pre-distortion methods may be employed to increase overalloptical power that may be launched into a fiber string 1604, which mayconnect one or more lasers 1600 to proximal circulator 310. Light pulse1602 may travel from proximal circulator 310 through first fiber opticcable 304 to WDM 404, which may be attached to a Raman Pump 402 at theopposite end, and to umbilical line 126. Light pulse 1602 may travel todistal circulator 312 in umbilical line 126 and the length of downholefiber 128. Any residual Raman amplification may be reflected back byFiber Bragg Grating 500 that has been constructed to reflect theparticular wavelengths used by the Raman Pump and transmit all others.The backscattered light from the downhole fiber 128 may travel back todistal circulator 312 and then up second fiber optic cable 308 to adedicated. interrogator receiver arm.

In examples, the dedicated interrogator receiver arm may allowinterrogator 124 to selectively receive backscattered light fromdifferent portions along the length of a fiber optic cable, as seen inFIGS. 10 and 11. For example, interrogator receiver aim may consist of adedicated amplifier 1608 that may selectively amplify the backscatteredlight from downhole fiber 128, a second region of the fiber optic cable,using a higher amplification factor than the dedicated amplifier 1608used to selective amplify the backscattered light received from firstfiber optic cable 304, a first region of the fiber optic cable. Gauges1612 may have gauge lengths employed in the two dedicated interrogatorreceiver arms may differ (e.g., also described in FIG. 2). Finally, eachdedicated interrogator receiver arm may be equipped with receivers 1606that are optimized according to certain characteristics of theinterferometric signals corresponding to the backscattered lightreceived from the two fiber sensing regions. Note that although FIG. 16only shows two dedicated interrogator receiver arms for each sensingfiber regions, it is not intended to be limited to such and may beextended to an arbitrary number of dedicated interrogator receiver aims,where each receiver aim receives and processes the backscattered lightsignal of a single sensing fiber region of downhole fiber 128.

FIG. 16 further illustrates example inputs 1610 for piezoelectric (PZT)devices. In examples, PZT devices functionally allow dynamic stretching(straining) of optical fibers, which may be embodied in coiled formaround the PZT, attached thereto, resulting in optical phase modulationof light propagating along the attached optical fiber. The PZT elementsare excitable via electrical signals from any electronic signalinformation generating source thus allowing information to be convertedfrom electrical signals to optical phase modulated signals along theoptical fiber attached thereto. Without limitation, PZT devices attachedto input 1610 may be a GPS receiver, seismic controller, hydrophone,and/or the like.

The systems and methods for a DAS system within a subsea environment mayinclude any of the various features of the systems and methods disclosedherein, including one or more of the following statements.

Statement 1. A distributed acoustic system (DAS) may comprise aninterrogator, an umbilical line attached at one end to the interrogator,and a downhole fiber attached to the umbilical line at the end oppositethe interrogator.

Statement2. The DAS of statement 1, wherein the interrogator furthercomprises a Raman Pump.

Statement 3. The DAS of statements 1 or 2, wherein the interrogatorfurther comprises a proximal circulator and a Raman Pump located betweenthe proximal circulator and the umbilical line.

Statement 4. The DAS of statements 1-3, wherein the DAS is disposed in asubsea system operation of one or more wells and the umbilical lineattaches to the downhole fiber at a fiber connection.

Statement 5. The DAS of statement 1, wherein the first fiber optic cableand the second fiber optic cable are connected to a distal circulator.

Statement 6. The DAS of statement 5, wherein the first fiber optic cableand the second fiber optic cable are different lengths.

Statement 7. The DAS of statements 1-4 or 7, further comprising aproximal circulator and a distal circulator and wherein one or moreremote circulators form the proximal circulator or the distalcirculator.

Statement 8. The DAS of statement 7, further comprising at least oneFiber Bragg Grating attached to the proximal circulator or the distalcirculator.

Statement 9. The DAS of statement 7, wherein the interrogator isconfigured to receive backscattered light from a first sensing regionand a second sensing region.

Statement 10. The DAS of statements 1-4, 7, or 8, wherein aninterrogator receiver arm is configured to receiver backscattered lightfrom the first sensing region or the second sensing region.

Statement 11. The DAS of statement 10, further comprising an opticalamplifier assembly, wherein the optical amplifier is attached to a firstoptical cable or a second optical cable at a proximal circulator.

Statement 12. The DAS of statement 10, wherein the distal opticalamplifier assembly is attached to a first optical cable of a secondoptical cable a distal circulator.

Statement 13. The DAS of statement 1-4, 7, 8, or 10, further comprisingat least one Fiber Bragg Grating that is attached between the umbilicalline and the downhole fiber.

Statement 14. The DAS of statement 13, wherein the at least one FiberBragg Grating is configured for a selected wavelength.

Statement 15. A method for optimizing a sampling frequency may compriseidentifying a length of a fiber optic cable connected to aninterrogator; identifying one or more regions on the fiber optic cablein which a backscatter is received; and optimizing a sampling frequencyof a distributed acoustic system (DAS) by identifying a minimum timeinterval that is between an emission of a light pulse such that at nopoint in time the backscatter arrives back at the interrogator thatcorresponds to more than one spatial location along a sensing portion ofthe fiber optic cable.

Statement 16. The method of statement 15, wherein the fiber optic cablecomprises an umbilical line connected to a downhole fiber through afiber connection.

Statement 17. The method of statements 15 or 16, further comprisingdetermining an optical energy of the backscatter.

Statement 18. The method of statements 15-17, wherein the fiber opticcable comprises an umbilical line and the umbilical line comprises afirst fiber optic cable and a second fiber optic cable both attached toa distal circulator.

Statement 19. The method of statements 15-18, wherein the interrogatorcomprises one or more lasers.

Statement 20. The method of statement 19, wherein the interrogatorcomprises one or more receivers.

Although the present disclosure and its advantages have been describedin detail, it should be understood that various changes, substitutionsand alterations may be made herein without departing from the spirit andscope of the disclosure as defined by the appended claims. The precedingdescription provides various examples of the systems and methods of usedisclosed herein which may contain different method steps andalternative combinations of components. It should be understood that,although individual examples may be discussed herein, the presentdisclosure covers all combinations of the disclosed examples, including,without limitation, the different component combinations, method stepcombinations, and properties of the system. It should be understood thatthe compositions and methods are described in terms of “comprising,”“containing,” or “including” various components or steps, thecompositions and methods can also “consist essentially of” or “consistof” the various components and steps. Moreover, the indefinite articles“a” or “an,” as used in the claims, are defined herein to mean one ormore than one of the element that it introduces.

For the sake of brevity, only certain ranges are explicitly disclosedherein. However, ranges from any lower limit may be combined with anyupper limit to recite a range not explicitly recited, as well as, rangesfrom any lower limit may be combined with any other lower limit torecite a range not explicitly recited, in the same way, ranges from anyupper limit may be combined with any other upper limit to recite a rangenot explicitly recited. Additionally, whenever a numerical range with alower limit and an upper limit is disclosed, any number and any includedrange falling within the range are specifically disclosed. Inparticular, every range of values (of the form, “from about a to aboutb,” or, equivalently, “from approximately a to b,” or, equivalently,“from approximately a-b”) disclosed herein is to be understood to setforth every number and range encompassed within the broader range ofvalues even if not explicitly recited. Thus, every point or individualvalue may serve as its own lower or upper limit combined with any otherpoint or individual value or any other lower or upper limit, to recite arange not explicitly recited.

Therefore, the present examples are well adapted to attain the ends andadvantages mentioned as well as those that are inherent therein. Theparticular examples disclosed above are illustrative only, and may bemodified and practiced in different but equivalent manners apparent tothose skilled in the art having the benefit of the teachings herein.Although individual examples are discussed, the disclosure covers allcombinations of all of the examples. Furthermore, no limitations areintended to the details of construction or design herein shown, otherthan as described in the claims below. Also, the terms in the claimshave their plain, ordinary meaning unless otherwise explicitly andclearly defined by the patentee. It is therefore evident that theparticular illustrative examples disclosed above may be altered ormodified and all such variations are considered within the scope andspirit of those examples. If there is any conflict in the usages of aword or term in this specification and one or more patent(s) or otherdocuments that may be incorporated herein by reference, the definitionsthat are consistent with this specification should be adopted.

What is claimed is:
 1. A distributed acoustic system (DAS) comprising:an interrogator; an umbilical line comprising a proximal circulatorconnected to a distal circulator by a first fiber optic cable and asecond fiber optic cable, and wherein the umbilical line is attached atone end to the interrogator; and a downhole fiber attached to theumbilical line at the end opposite the interrogator.
 2. The DAS of claim1, wherein the interrogator further comprises a Raman Pump.
 3. The DASof claim 1, wherein the DAS further comprises a Raman Pump locatedbetween the proximal circulator and the umbilical line.
 4. The DAS ofclaim 1, wherein the DAS is disposed in a subsea system operation of oneor more wells and the umbilical line attaches to the downhole fiber at afiber connection.
 5. (canceled)
 6. The DAS of claim 1, wherein the firstfiber optic cable and the second fiber optic cable are differentlengths.
 7. The DAS of claim 1, wherein one or more remote circulatorsform the proximal circulator or the distal circulator.
 8. The DAS ofclaim 7, further comprising at least one Fiber Bragg Grating attached tothe proximal circulator or the distal circulator.
 9. The DAS of claim 7,wherein the interrogator is configured to receive backscattered lightfrom a first sensing region and a second sensing region.
 10. The DAS ofclaim 9, wherein an interrogator receiver arm is configured to receiverbackscattered light from the first sensing region or the second sensingregion.
 11. The DAS of claim 10, further comprising an optical amplifierassembly, wherein the optical amplifier assembly is attached to thefirst fiber optic cable or the second fiber optic cable at the proximalcirculator.
 12. The DAS of claim 10, wherein the optical amplifierassembly is attached to the first fiber optic cable or the second fiberoptic cable at the remote circulator.
 13. The DAS of claim 1, furthercomprising at least one Fiber Bragg Grating that is attached between theumbilical line and the end of the downhole fiber.
 14. The DAS of claim13, wherein the at least one Fiber Bragg Grating is configured for aselected wavelength.
 15. A method for optimizing a sampling frequencycomprising: identifying a length of a fiber optic cable connected to aninterrogator; identifying one or more sensing regions on the fiber opticcable in which a backscatter is received; and optimizing a samplingfrequency used for each of the one or more sensing regions of adistributed acoustic system (DAS) by identifying a minimum time intervalthat is between an emission of a light pulse such that at no point intime the backscatter arrives back at the interrogator that correspondsto more than one spatial location along a sensing portion of the fiberoptic cable, and wherein the wavelength of the light pulse is the samefor the one or more sensing regions.
 16. The method of claim 15, whereinthe fiber optic cable comprises an umbilical line connected to adownhole fiber through a fiber connection.
 17. The method of claim 15,further comprising determining an optical energy of the backscatter. 18.The method of claim 15, wherein the fiber optic cable comprises anumbilical line and the umbilical line comprises a first fiber opticcable and a second fiber optic cable both attached to a distalcirculator.
 19. The method of claim 15, wherein the interrogatorcomprises one or more lasers.
 20. The method of claim 19, wherein theinterrogator comprises one or more receivers.